The present invention relates to methods of manufacturing surface wave dispersive filters using the reflection of acoustic waves from variable pitch networks slanted with respect to the propagation of these acoustic waves.
These filters are known under the name of "reflective array compressors" abbreviated to RAC. The product of the pass band multiplied by the signal time, called BT, of these filters is very high, which forms the advantage thereof.
An RAC corresponding to the state of the art is shown in FIG. 1. It includes, on a piezoelectric substrate 1, an integerdigitated transmitting transducer 10 which, from the signal to be compressed, sends an acoustic wave in the direction of a first reflecting network 100. This network includes a series of discontinuities in the form of lines slanted substantially at 45.degree. with respect to the axis of propagation Z of the acoustic wave. These discontinuities, as well as transducer 10 and transducer 11 which will be discussed further on, are formed by etchings or metallizations on the surface of the substrate.
The acoustic wave is reflected by network 100 substantially at 90.degree. towards a second network 110 identical to the first one but symmetric with respect to axis Z. The acoustic wave is then reflected substantially at 90.degree. from the second network in the direction opposite axis Z.
It is then picked up by an output transducer 11 which it energizes so as to give an electric output signal.
The spacing between the discontinuities in the two reflecting networks is variable as a function of the distance to the input and output transducers. Thus, the reflecting power is variable with the frequency and depending on this frequency the acoustic wave will travel over a path such as 12 whose length, and so duration, will depend on the frequency.
The overall transfer function of this device corresponds to the product of the transfer functions of the input and output transducers and of the reflecting networks and causes compression of the pulse applied to the input transducer.
Such a device, because of different parasitic phenomena, does not then give directly the desired amplitude and phase response in the useful band.
To correct the amplitude and phase defects, it is known to correct the response of the RAC by means of a conventional electric filter, of RLC type, connected in series with this RAC. The transfer function of the RAC is measured and from this a filter is synthesized using known methods having a transfer function for correcting that of the RAC.
In this case at the level of the correction filter known drawbacks are added such as insertion loss, space required and the influence of the different drifts, particularly temperature drifts.
It is further known, but solely in so far as the phase error is concerned, to correct this latter by placing between the two reflecting networks 100 and 110 a conducting film of variable width and thickness so as to modify the propagation speed for each path of the waves and so for each frequency. This method however does not correct the fluctuations of rapid errors.